Does anyone out their know how to deal with feedback problems? I have a Asturias classical guitar made in Japan, not a very good instrument but all I have presently. I had a Fishman pic-up installed that has a built in equalizer with notch and frequency knobs to handle feedback, but only works at lower volumns. I don't know how good this pic-up is. Someone told me to try putting a sponge in my guitar. Anyone have anymore suggestion?

Sincerly,

Greg

Tim posted on Monday, 13 March, 2006 - 08:37 pm

You could try turning the volume down on your amp!

Seriously the problem of feedback is caused by sound from the speakers creating sympathetic vibrations in the top of the guitar, that's why EQ can get rid of feedback. Try moving your amp (2-3 metres) further away from the guitar and angling it (or yourself) away at angle of 40-45 degrees, with the amp on the opposite side to your guitar's body - i.e. if you sit in a 'classical position' with the guitar on your left knee, have the amp to your right.

If your amp's overdriven a more powerful amp at the same volume may help.

Putting a sponge (or a selection of ladies' hosiery) in the guitar can do the trick, especially if you want to sound like you're playing a guitar with a sponge in it...

Finally, save up and buy a better guitar with built in pick-up and onboard EQ.

Tim posted on Monday, 13 March, 2006 - 09:17 pm

BTW when Richard asked for feedback in another posting I don't think this is what he had in mind. Years ago Bob Dylan's manager asked a journalist who'd been in the audience if there was any feedback on the new songs - "well" came the reply, "there's a lot of whistling and distortion when he gets too close to the mike..."

Tricky posted on Tuesday, 14 March, 2006 - 01:37 pm

Positive feedback can't be effectively controlled from outside of the amps circuitry. All amps have some internal negative feedback circuitry (feeding a controlled out-of-phase signal back to the input) to suppress high frequency noise, hence reducing the drain on the power supply, making everything more stable and reducing distortion. Negative feedback loops can be "tuned".

Positive feedback tends to occur at certain frequencies, depending on the ambient conditions. The only real way to control it is to "tune the amp for the room". I don't know offhand how they do it at large gigs, but would imagine the amps have their own equivalent of a "positive feedback frequency range suppressor equaliser" which is set up before the show. As usual it comes down to the depth of your pockets.

Tricky posted on Thursday, 16 March, 2006 - 09:48 pm

Just remembered something from my early years about feedback and digital amps. (the above post was for analogue). A digital amps' feedback can be controlled by introducing a (bucket brigade) delay. A 1/20th second delay won't be noticeable to us mortals, but will only result in feedback at a single frequency - which (digital control being what it is) can be completely eliminated.

Not much help, I know, but if you ever go for some different equipment, you'll have an idea of what to look out for.

Reading between the lines (and I must confess electrical circuitry is beyond me so that's as much as I can manage) what you've just posted Tricky, is to do with the electrics. The problem with amplifying an acoustic guitar is an acoustic problem. A guitar works by vibrations from the strings being transferred via the bridge to the guitars top: this then causes the air inside the guitar to vibrate, and the sound waves created travel out via the sound hole. Many luthiers and guitar factories tune the guitar's top to a specific note as this gives different instruments a uniform sound.

But when the guitar top is hit by its own amplified sound, the whole top is put into a sympathetic vibration, and that's what creates the feedback. Ironicaly better guitars with thinner tops are more prone to this, which is why they need better equalisation to cut it out of the signal. Most rock guitarists adopted solid body guitars to avoid this - that's why a real Les Paul is heavier than a cheapo lookalike.

Stuffing your guitar body with sponges, ladies' hose, old tee-shirts etc can elminate feedback by deadening the top's vibration. It deadens the sound too. The old trick of angling a combo-amp away from the guitar works by pointing the amplified sound away from the instrument. If you're entertaining your local shopping centre to earn a few bob, stick the amp in front of you.

Tricky posted on Friday, 17 March, 2006 - 07:56 pm

You're right Tim. The point I'm rather badly trying to put over is that the resonant frequency range can be suppressed within the amps circuitry. The simplest -ve feedback loops are fairly linear ie:- 0.1% of the inverted output fed back, but they don't need to be. You can increase the %feedback depending on the frequency. (There's also the little matter of automatic gain control - more commonly abbreviated to AGC - not mentioned much these days).

A purely digital amps output can be made to be always (more or less) out-of-phase with the resonace in the guitar - effectively making the circuit, which in this case includes the guitar and loudspeakers, self suppressing.

Purely speculatively, (and a hint for amp manufacturers) a digital amp could send out a pure note outside of we mortals audio range that could be used to control the out-of-phase condition. Wait a minute... this might encourage the pubs karaoke singers to turn up the volume....

All this komplikated tech talk has made me thirsty - you're not playing in the Ordinance Arms tonight, are you Greg ?...

Tim posted on Friday, 17 March, 2006 - 09:43 pm

OK, we're in agreement.

The principal function of socks and sponges in your guitar is to deaden the top. It means when you practice at home no-one else can hear you...

If you play in restaurants or busk, careful positioning of your amp angled away from the guitar can reduce feedback.

Feedback can be eliminated most effectively electronically - but that costs. It costs whether it's part of the guitars pick-up or at the amp stage.

There's some suggestions that a soundhole pickup is best to reduce feedback - tho' of course it won't work with nylon strings (see the iron filings in another discussion). It also has the least natural sound (listen to the Pentangle's 'Sweet Child' if you want recorded proof). Choosing between a piezo transducer (contact pickup) and an internal mike doesn't seem to effect feedback, tho' the mike has the most natural sound. The most sophisticated systems blend both transducer and internal mike - again to do it properly money, money, money...

Try an internal mike - at one time Dave Swarbrick swore by a Tandy lapel mike clipped inside his fiddle. Cheap.

As to getting a digital amp to emit an inaudible signal - "who let the dogs out!!"

Tricky posted on Saturday, 18 March, 2006 - 06:54 pm

A queue of dogs outside of a nightclub ? - a common sight in Oldham :-)